Method of High-Performance Flash Memory Data Transfer

ABSTRACT

A flash memory system including a flash memory device and a controller, operable according to an advanced data transfer mode is disclosed. The flash memory device is operable both in a “legacy” mode, in which read data is presented by the memory synchronously with each cycle of a read data strobe from the controller, and in which input data is latched by the memory synchronously with each cycle of a write data strobe from the controller. In the advanced mode, which can be initiated by the controller forwarding an initiation command to the memory, the flash memory itself sources the read data strobe at a higher frequency, for example at twice the frequency, of that available in the normal mode. In the advanced mode, the input data is presented by the controller synchronously with a higher frequency write data strobe than is available in the normal mode. The voltage swing of the data and control signals is reduced from conventional standards, to reduce power consumption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority, under 35 U.S.C. §120, to copendingapplication Ser. No. 11/379,895 filed Apr. 24, 2006, entitled “Method ofHigh-Performance Flash Memory Data Transfer”, and Ser. No. 11/379,910,filed Apr. 24, 2006, entitled “High-Performance Flash Memory DataTransfer”, both commonly assigned with this application, and is relatedto my copending and commonly assigned application Ser. No. ______(SDK0873.001US), filed on the same day as this application, entitled“High-Performance Flash Memory Data Transfer”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of flash memory devices, and is morespecifically directed to data communications between flash memorydevices and memory controllers in electronic systems.

As well known in the art, “flash” memories are electrically-erasablesemiconductor memory devices that can be erased and rewritten inrelatively small blocks, rather than on a chip-wide or large-block basisas in previous electrically-erasable programmable read-only memory(EEPROM) devices. As such, flash memory has become especially popularfor applications in which non-volatility (i.e., data retention afterremoval of power) of the stored data is essential, but in which thefrequency of rewriting is relatively low. Examples of popularapplications of flash memory include portable audio players, “SIM” cardstorage of telephone numbers and phone activity in cellular telephonehandsets, “thumbkey” removable storage devices for computers andworkstations, storage devices for digital cameras, and the like.

An important recent advance in semiconductor non-volatile memorytechnology is the arrangement of the flash memory cells as “NAND” memoryrather than as “NOR” memory. As known in the art, NOR flash memoryrefers to the conventional arrangement of a column of memory cells inparallel between a bit line and a source line. Access of a specific cellin a NOR column is made by driving its word line (control gate) activewhile holding the other cells in the column off, so that the currentbetween the bit line and source line is determined by the state of theaccessed cell. Memory cells in a column of NAND memory, on the otherhand, are connected in series between the bit line and the source line.Accessing of a specific cell in a NAND column thus requires turning onall of the cells in the column with active word line levels, andapplying an intermediate word line level to the cell to be accessed,such that the current between the bit line and source line is, again,determined by the state of the accessed cell. As well known in the art,the chip area required per bit of NAND flash memory is much reduced fromthe area per bit of NOR flash memory, primarily because fewer conductors(and therefore contacts) are required for a column of NAND memoryrelative to NOR memory; in addition, access transistors can be sharedamong a large number of cells in the NAND arrangement. Additionally,conventional NAND flash memory is conveniently accessed serially, forexample by sequentially accessing cells along the columns, rather thanas a random access memory as in the case of NOR memory. NAND memory isthus especially well-suited for music and video storage applications

Another important recent advance in the field of flash memory isreferred to in the art as the multilevel program cell (MLC). Accordingto this approach, more than two data states are made possible for eachmemory cell, simply by more finely controlling the programming of thecell. In conventional binary data storage, each memory cell isprogrammed into either a “0” or a “1” state. Reading of such binarycells is accomplished by applying a single control voltage to thecontrol gate of the addressed memory cell so that the transistorconducts if programmed to a “1” state, but remains off in the “0” state;sensing of the conduction through the addressed memory cell thus returnsthe programmed state of the cell. In contrast, according to a typicalexample of the MLC approach, four possible states are defined for eachmemory cell, typically corresponding to binary values 00, 01, 10, 11. Ineffect, the two intermediate states correspond to two levels of partialprogramming of the cell between the fully erased and fully programmedstates. Some implementations of MLC flash memory with up to eightpossible states, or three binary bits, per cell are known. The abilityto store two or three bits of data on each memory cell immediatelydoubles or triples the data capacity of a flash memory chip. Examples ofMLC flash memory cells and memories including such MLC cells aredescribed in U.S. Pat. No. 5,172,338, and U.S. Pat. No. 6,747,892 B2,both commonly assigned herewith and incorporated herein by thisreference.

The combination of MLC technology with the efficiencies of NAND flashmemory architectures has resulted in significantly reduced cost per bitfor semiconductor non-volatile storage, as well as improved systemreliability, and a higher data capacity and system functionality for agiven form factor. However, despite these important improvements, thedata transfer rates to and from conventional flash memory devices havenot kept pace. Certain modern applications of flash memory areespecially sensitive to data transfer rates, especially as the datacapacity increases. For example, the resolution of high-performance,professional level, digital still cameras now can exceed 10 megapixels,for which the advances of MLC NAND flash memory technology are welcome.However, the “shutter lag” between successive image captures depends onthe data transfer rate of the image data from the sensor into flashmemory. This delay time between images (which, to the camera user, isconsidered as an independent parameter, not dependent on imageresolution) is becoming a critical factor in these cameras. Especiallyas the image resolution continues to increase, it has been observed thatconventional data transfer times are not adequate to achieve the desireddelay time between images. Nor are the data transfer times into and outof conventional flash memory competitive with those of modern magneticdisk drives, which is of course another desirable new application forflash memory. Accordingly, in order for flash memory to meet the needsof modern high-performance digital still cameras, or to serve assolid-state mass storage in modern high-performance electronic systems,it will become necessary to achieve much higher data transfer rates toand from flash memory devices.

An example of a conventional data transfer approach for flash memoriesis described in the datasheet 2GBIT (256M×8 BITS) CMOS NAND E²PROM, partnumber TH58NVG1S3AFT05 (Toshiba, 2003). This conventional approachinvolves an eight-bit data bus, with one bit presented on each dataoutput per cycle of a read enable clock, synchronous with the fallingedge of that read enable clock. Also as described in that datasheet,this conventional approach involves a 3.3 volt logic standard, such thatthe minimum high logic level output voltage (V_(OH)) is 2.4 volts andthe maximum low logic level output voltage (V_(OL)) is 0.4 volts. Thisdevice provides a maximum data rate of 20 MHz. It is believed that thisdata rate is not an adequate data rate for mass storage in personalcomputer systems, and as such these conventional flash memories wouldnot be suitable for disk drive replacement.

By way of background, some conventional dynamic random access memories(RAMs) implement so-called “double data rate”, or “DDR”, data transfertechniques. As known in that art, DDR data transfer involves thetransfer of one or more data bits (depending on the number of bus lines)synchronously with both the rising and falling edges of thecorresponding data strobe, or clock. DDR data transfer thus communicatesdata at twice the data rate of conventional synchronous data transfer,which is synchronous with only one of the clock edges (rising or fallingedge). In addition, conventional DDR dynamic RAMs utilizesource-synchronous data strobes, in which the RAM device itselfgenerates the data strobe for reads from the memory (while the externalcircuitry generates the data strobe for writes to the memory). However,this doubling of the input/output switching rate increases the powerconsumption of data transfer, approaching twice that of single-data ratecommunications.

Power consumption in modern electronic systems is a substantial concern,however, and the driving of buses and conductors in transferring dataamong integrated circuit devices in a system is a significantcontributor to overall system power consumption. As is fundamental inthe art, the power consumption of output driver circuits, for drivingexternal conductors, relates directly to the switching rate of digitalsignals to be driven. Increasing the data transfer rate to approachthose of modern magnetic disk drives, as mentioned above, thus willrequire a corresponding increase in the power consumed by such datatransfer, keeping all other parameters equal. This increased powerconsumption requires larger driver and receiver devices, improved heatdissipation in system applications, and the like, all of which add costto the overall system. Even if these changes are made, the increasedpower consumption from high-speed data transfer is undesirable forportable electronic systems, such as digital cameras, laptop computersand workstations, wireless telephone handsets, personal digital audioplayers, and similar battery-powered devices.

By way of further background, a communications protocol known as UltraDMA Mode is known in the art, for communications to and from a flashmemory card, such as a COMPACT FLASH, or CF+, flash memory card. FIG. 1illustrates such a conventional flash memory card, constructed andoperating according to the well-known standard CF+ and CompactFlashSpecification Revision 3.0 (CompactFlash Association, 2004). As shown inFIG. 1, flash memory card 2, which in this example is constructed as aCOMPACT FLASH storage card according to this standard, contains one ormore flash memory modules 2, and single chip memory controller 4. Flashmemory module 4 communicates data to and from memory controller 6 overbus data_I/O, and receives and issues control signals to and from memorycontroller 6 over control bus ctrl. In this example, the data transferapproach described in the above-referenced Toshiba datasheet correspondsto these communications over the data_I/O and ctrl buses between flashmemory module 4 and memory controller 6. Memory controller 6communicates with a host device (e.g., digital camera, digital audioplayer, personal computer, etc.) over host interface HOST_IF. Theabove-referenced CF+ and CompactFlash Specification describescommunications over host interface HOST_IF, including according to theUltra DMA Mode (“UDMA”). As described in that specification, UDMAcommunications are carried out in a special operating mode, initiated bythe driving of a signal on a control line (UDMARQ) by the agent (host ormemory card 2) that desires such communication. Also as described inthat specification, UDMA data transfers are source-synchronous, in thatthe agent (memory card 2 or host system) that is placing the data ontobus HOST_IF is also issuing the data strobe signal. In addition, also asdescribed in that specification, both rising and falling edges of thestrobe signal are used in the transfer of data under the UDMA mode ofoperation.

However, it has been observed, in connection with this invention, thateven with the UDMA mode for the host interface in the flash card of FIG.1, the data transfer rate between the memory module 4 and memorycontroller 6 will limit the overall performance of memory card 2.However, the speeding up of data transfer at that interface according toconventional techniques will also greatly increase power consumptionwithin memory card 2. In addition, it is known in the art thatmodification to input/output interfaces of memory integrated circuitswill greatly limit the usability of such integrated circuits, addingcost from the standpoint of inventory control and design overhead.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a method of flashmemory module having a high-performance data transfer mode, for datatransfer to and from a memory controller.

It is a further object of this invention to provide such a method inwhich data transfer according to the high-performance mode consumespower at a rate that is not substantially greater than conventional datatransfer.

It is a further object of this invention to provide such a method inwhich “legacy” data communications can also be carried out, to providebackward-compatibility with conventional data transfer standards.

It is a further object of this invention to provide such a method inwhich data skew in the high-performance data transfer mode is minimized.

Other objects and advantages of this invention will be apparent to thoseof ordinary skill in the art having reference to the followingspecification together with its drawings.

The present invention may be implemented into a method of operating aflash memory device having a multiple-mode data interface. In a legacymode, the data interface presents or receives data synchronously with anexternally-generated data strobe, with one bit per conductorcommunicated in each cycle of the strobe. In an advanced mode, the datainterface is source-synchronous, with a data bit or word synchronouswith either a rising or falling edge of a strobe signal that is at twicethe frequency of the legacy mode strobe. A reduced voltage swing isprovided for the advanced mode, to reduce power consumption. Uponinvocation of the advanced mode for data transfers, the legacy operatingmode continues to be used for command and control communications; datatime-out and other automated control functions are provided for theadvanced operating mode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an electrical diagram, in block form, of a conventional memorycard.

FIG. 2 is an electrical diagram, in block form, of a memory moduleconstructed according to the preferred embodiment of the invention.

FIG. 3 is an electrical diagram, in block form, of the memory module ofFIG. 2 implemented into a system or subsystem in combination with asingle-chip memory controller, according to the preferred embodiment ofthe invention.

FIGS. 4 a through 4 d are timing diagrams illustrating the operation ofthe flash memory module of FIGS. 2 and 3 in a normal operating mode, andin the communication of commands, according to the preferred embodimentof the invention.

FIGS. 5 a and 5 b are flow diagrams illustrating the operation ofadvanced mode read data transfers and write data transfers,respectively, according to the preferred embodiments of the invention.

FIGS. 6 a through 6 e are timing diagrams illustrating signals involvedin the operations of FIGS. 5 a and 5 b according to a first preferredembodiment of the invention.

FIG. 7 is a flow diagram illustrating the operation of advanced modedata transfers according to a second preferred embodiment of theinvention.

FIGS. 8 a through 8 e are timing diagrams illustrating signals involvedin the operations of FIGS. 5 a and 5 b according to a second preferredembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in connection with its preferredembodiment, namely as implemented into a flash memory module, and asubsystem including such a flash memory module, and method of operatingthe same. More specifically, this exemplary flash memory module isdescribed as a multi-level cell (MLC) flash memory of the NAND type, asit is contemplated that this invention will be particularly useful inconnection with such flash memories, in order to enable the use ofsolid-state non-volatile memory for mass data storage in computersystems. However, it is contemplated that this invention will be usefuland beneficial in other applications involving non-volatile solid-statememories of various types. Accordingly, it is to be understood that thefollowing description is provided by way of example only, and is notintended to limit the true scope of this invention as claimed.

FIG. 2 illustrates an exemplary construction of flash memory device (ormodule) 10 constructed according to a preferred embodiment of theinvention. It is contemplated that flash memory device 10 will typicallybe constructed into a single integrated circuit, and as such may beinterfaced with any one of a number of memory controllers or memorycontroller logic, as will be described in further detail below. It isalso contemplated that the architecture of flash memory device 10illustrated in FIG. 2 is merely an example presented for purposes ofcomprehending this invention, and that those skilled in the art havingreference to this specification can readily realize this invention inconnection with flash memory devices of architectures varying from thatshown in FIG. 2.

The storage capability of flash memory device 10 resides in flash memoryarray 12. Array 12 includes electrically programmable and erasablememory cells arranged in rows and columns, as known in the art. While asingle array 12 is shown in FIG. 2, it is of course contemplated thatarray 12 may be realized as multiple sub-arrays, each having a separateinstance of peripheral circuitry, such as part or all of the address,data, or control circuitry described in further detail below relative tothe example of FIG. 2. It is contemplated that those skilled in the arthaving reference to this specification will be readily able to realizethis invention in connection with such multiple sub-array architectures.In this example, the memory cells of array 12 are floating-gatemetal-oxide-semiconductor (MOS) transistors, constructed so that eachsuch transistor, corresponding to one memory cell, can be electricallyprogrammed and also electrically erased. According to the preferredembodiment of the invention, the memory cells of array 12 aremulti-level cells (MLC), in that they may be programmed to more than twodata states (i.e., to any of more than two threshold voltages), so thateach such cell stores a multiple-bit digital value. Also according tothis preferred embodiment of the invention, as will be evident from thefollowing description, these memory cells are preferably arranged in thewell-known NAND fashion, such that the cells are typically not randomlyaccessed but are rather accessed serially, as useful for mass storageapplications. Of course, this invention may also be used in connectionwith binary memory cells (i.e., storing only a single digital bit), andin connection with NOR arrangements of the memory cells.

According to this preferred embodiment of the invention, commoninput/output terminals I/O1 through I/On are provided, and connected toinput/output control circuit 20. As known in the art for NAND type flashmemories, the operation of flash memory device 10 is controlled in largepart by the receipt and execution of commands, communicated as digitalwords over input/output terminals I/O1 through I/On, and executed bycontrol logic 18. As such, input/output control circuit 20 receivescontrol commands, address values, and input data, and presents statusinformation and output data, via its driver and receiver circuitry thatcommunicate with input/output terminals I/O1 through I/On. It iscontemplated that the number n of input/output terminals I/O1 throughI/On will generally be eight or sixteen, although, of course, any numberof such terminals may be provided. In addition, input/output controlcircuit 20 receives power supply voltage V_(cc-R) and drivesinput/output terminals I/O1 through I/On with logic levels based uponthat voltage. According to this preferred embodiment of the invention aswill be described in detail below, this power supply voltage V_(cc-R) isat a lower voltage than that used in conventional flash memory devices,so that the power consumption resulting from data transfers atinput/output terminals I/O1 through I/On is reduced, even at higherswitching rates. Control logic 18 also receives this power supplyvoltage V_(cc-R) based upon which it will drive output control signalsat the lower voltages from read enable terminal RE_, among others.

Input/output control circuit 20 forwards command information to commandregister 24, for decoding and execution by control logic 18, as itcontrols the operation of flash memory device 10. Status information isstored by control logic 18 in status register 23, in the conventionalmanner. Address values received at input/output terminals I/O1 throughI/On by input/output control circuit 20 are buffered in address register22; the row portion of such addresses is decoded by row decoder 11 andthe column portion is decoded by column decoder 15 (each of whichtypically includes an address buffer), to effect selection of thedesired cell or cells in array 12, in the conventional manner.Input/output control circuit 20 is also in bidirectional communicationwith data register 14, via bus DATA_BUS, to forward data to be writtento data register 14, and to receive output data from data register 14,depending upon the direction of the data transfer to be executed.Control logic 18 also receives various direct control signals fromexternal to flash memory device 12, including, for example, lines forthe signals of chip enable CE_, command latch enable CLE, address latchenable ALE, write enable WE_, read enable RE_, and write protect lineWP_. As known in the art, the command latch enable CLE and address latchenable ALE signals indicate whether a command or address is beingpresented on input/output terminals I/O1 through I/On, while the writeenable WE_ and read enable RE_ signals serve as the data strobes inwrite and read operations, respectively.

According to this embodiment of the invention, the write enable WE_signal is an input to flash memory device 10. Accordingly, for thetransfer of data into flash memory device 10 via input/output terminalsI/O1 through I/On, the write data strobe conveyed as the write enableWE_ signal is always sourced by a device external to flash memory device10, typically by the source of the incoming data itself. However, alsoaccording to the preferred embodiment of this invention and as will bedescribed in further detail below, the read enable RE_ signal isbidirectional. In a normal operating mode, the external device that isthe destination of data being read from flash memory array 12 is thesource of the read data strobe, which is then conveyed as an input toflash memory device 10 as the read enable RE_ signal. In an advancedoperating mode according to the preferred embodiment of the invention,as will be described in further detail below, control logic 18 issuesthe read data strobe as the read enable RE_ signal, synchronous withdata read from flash memory array 12 and communicated via data register14, I/O control circuit 20 and input/output terminals I/O1 through I/On.

FIG. 3 illustrates an implementation of flash memory device (or module)10 into flash memory card 25, according to a preferred embodiment of theinvention. As shown in FIG. 3, flash memory card 25 includes at leastflash memory device 10 itself and also controller 30. Controller 30provides and manages an external interface HOST_IF to a host system,such as a high-performance digital camera, a personal computer, or aportable device such as a digital audio player or cellular telephonehandset or the like; interface HOST_IF may also correspond to a set ofexternal terminals of flash memory card 25 constructed as a generalpurpose card that may be inserted into any one of a wide range of hostsystems, as known in the art. It is contemplated that interface HOST_IFmay operate according to a conventional standard interface as known inthe art currently, or as may be developed in connection with futureflash memory interface standards or proprietary interface protocols. Asmentioned above, it is contemplated that this invention will beespecially beneficial in providing high speed data transfer, such as inthe data transfer rate critical application of a high-performancedigital still camera. It is further contemplated that the high datatransfer rates provided by this invention can also enable the use offlash memory as a solid-state mass storage device in a personalcomputer, replacing magnetic disk drives. As such, it is contemplatedthat interface HOST_IF will best have high-speed data transfercapability, for example as contemplated by the UDMA standard mentionedabove in the Background of the Invention.

As shown in FIG. 3, flash memory device 10 is coupled to controller 30in a manner consistently with the terminals shown in FIG. 2. In thisregard, an input/output bus is formed by signal lines I/O1 through I/On,corresponding to similarly named terminals of flash memory device 10. Acontrol bus CTRL couples controller 30 to flash memory device 10, andincludes signal lines connected to the ALE, CLE, WP_, and CE_ terminalsshown in FIG. 2. It is contemplated that other control lines andterminals may also be provided for communication between flash memorydevice 10 and controller 30, and as such control bus CTRL is illustratedas a bidirectional bus, even though the ALE, CLE, WP_, and CE_ terminalsare shown in FIG. 2 as inputs to flash memory device 10.

FIG. 3 illustrates two control lines, RE_ and WE_, separately fromcontrol bus CTRL, for the sake of clarity of this description. Accordingto this embodiment of the invention, line WE_ carries the data strobe inwrite operations (data written from controller 30 to flash memory device10), and as such is connected to terminal WE_ of flash memory device(FIG. 2). According to this preferred embodiment of the invention, thedata strobe on line WE_ is sourced by controller 30 in each of theoperating modes. Line RE_ carries the data strobe for read operations(data read from flash memory 10 and communicated to controller 30), andas such is connected to terminal RE_ of flash memory device 10 (FIG. 2).As mentioned above, according to this preferred embodiment of theinvention, line RE_ is bidirectional, with the source of the read datastrobe depending on the current operating mode of flash memory device10. In a normal operating mode, controller 30 sources the read datastrobe, in response to which flash memory device 10 maintains as validdata it presents on signal lines I/O1 through I/On. In an advancedoperating mode according to the preferred embodiment of the invention,flash memory device 10 sources the read data strobe on line RE_ for datatransfer from flash memory device 10 to controller 30. As will bedescribed in further detail below, commands communicated by controller30 over signal lines I/O1 through I/On are synchronous with a read datastrobe source on signal line RE_, regardless of the operating mode inwhich flash memory device 10 is transferring data to controller 30.

It is contemplated that controller 30 will be constructed substantiallyaccording to conventional flash memory controller architectures, asknown in the art, modified as necessary to effect the operationsdescribed in this specification in connection with the initiation,operation, and termination of read operations in the advanced operatingmode of flash memory device 10 according to the preferred embodiment ofthe invention. It is also contemplated that the logic hardware, programinstructions, or combination thereof for implementing these advancedoperating mode functions within controller 30 will be apparent to thoseskilled in the art having reference to this specification. As such, itis further contemplated that those skilled readers will be readily ableto implement such modifications of controller 30, as best appropriatefor a particular realization, without undue experimentation.

And, also as shown in FIG. 3, power supply voltage V_(cc-R) is connectedto, and biases, each of flash memory device 10 and controller 25. Thispower supply voltage V_(cc-R) is at a lower voltage than that used inconventional flash memory devices and controllers, so that the powerconsumption resulting from data transfers and transitions overinput/output lines I/O1 through I/On and the various control lines isreduced, even at higher switching rates as will be described below. Aswill be discussed in further detail below in connection with aparticular example, this power supply voltage may be at a nominalvoltage of about 1.80 volts, within a range from about 1.60 volts toabout 2.00 volts, which is substantially lower than the conventionalstandard nominal power supply voltage of 3.30 volts, within aspecification range of between 2.70 volts and 3.60 volts.

Referring now to FIGS. 4 a through 4 e, the operation of flash memorydevice 10 in combination with controller 30 in memory card 25, accordingto a normal operating mode and also a command communication mode, willnow be described. It is contemplated that these operating modes willsubstantially correspond to conventional flash memory interfaceprotocols for modern flash memory devices, and as such these operatingmodes will serve as a “legacy” input/output protocol for flash memorydevice 10 according to the preferred embodiment of the invention.

FIG. 4 a illustrates the communication of a command from controller 30to flash memory device 10. As known in the art, and as will be describedin further detail below, modern flash memory devices operate in responseto specific commands that are issued by a controller, and that arecommunicated over the data input/output lines. As such, in this example,the communication of a command CMD is effected by controller 30 drivingcommand latch enable signal CLE to a high active state, and addresslatch enable signal ALE to a low inactive state, signifying that acommand, rather than an address, will be communicated on input/outputlines I/01 through I/On. Chip enable signal CE_ is taken active low,enabling flash memory device 10 in the conventional manner; as known inthe art, if multiple flash memory devices 10 are provided within card25, individual chip enable signals CE_can be used by controller 30 as itselects the desired one of the flash memory devices 10 forcommunication. The digital word presented by controller 30 oninput/output lines I/01 through I/On, corresponding to command CMD asshown in FIG. 4 a, is strobed by controller 30 issuing an active lowpulse on write enable line WE_; the rising edge of the pulse on lineWE_causes I/O control circuit 20 to receive and latch in the commandCMD, eventually reaching command register 24 (FIG. 2). Controller 30 canthen return the command latch enable signal CLE to an inactive lowstate, terminating the command operation. Of course, as known in theart, multiple word commands, or multiple single-word commands, can becommunicated sequentially in this manner, with command latch enable lineCLE held high for the duration of such communications.

One command that is communicated in the manner illustrated in FIG. 4 ais a command (e.g., command 00H for a read operation; command 10H for aserial data input program, or write, operation) indicating that a memoryaddress will be communicated by controller 30 to flash memory device 10.FIG. 4 b illustrates the timing of the communication of such an addressby controller 30 to flash memory device, in a normal and commandoperating mode according to the preferred embodiment of the invention.As such, the operation illustrated in FIG. 4 b follows the communicationof a command 00H, according to the sequence of FIG. 4 a, indicating theimminent transmission of a memory address in the next signal sequence.

A relatively wide range of commands can be communicated by controller 30to flash memory device 10 in this normal operating mode. The followingtable lists an exemplary command set in this preferred embodiment of theinvention:

Command Command code (hexadecimal) Serial data input 80 Auto program 10Read address input 00 Column address change during serial data 05 outputRead start 30 Read column address change E0 Auto block erase 60, D0 (twocycle command) ID read 90 Status read 70 Reset FF

Referring now to FIG. 4 b, the transmission of a memory address fromcontroller 30 to flash memory device 10 according to this preferredembodiment of the invention will be described. In this operation,controller 30 drives command latch enable signal CLE inactive low, anddrives address latch enable signal ALE high, indicating to flash memorydevice 10 that an address value will be communicated (rather than acommand value) on input/output lines I/O1 through I/On. The chip enablesignal CE_ is also driven active low, indicating that controller 30 isselecting flash memory device 10 as the recipient of this addressinformation. In this operation, controller 30 issues active low pulsesof the write enable signal WE_, each pulse indicating a portion of theaddress value then presented by controller 30 on input/output lines I/O1through I/On. In this embodiment of the invention, this addressinformation is synchronous with the rising edge (i.e., end of the activelow pulse) of write enable signal WE_, such that flash memory 10 can usethis edge to latch the then-current state of input/output lines I/O1through I/On into address register 22 (FIG. 2) as a portion of thedesired memory address. As evident in the example of FIG. 4 b, thememory address extends over multiple words (the width of which isdefined by the number n of input/output lines I/O1 through I/On). Inthis case, the memory address includes four address words ADD0 throughADD3 that are presented synchronously with successive active low pulsesof write enable signal WE_.

Following the communication of an address value as shown in FIG. 4 b,controller 30 can perform a write of data to flash memory device 10, ora read of data therefrom. FIG. 4 c illustrates the signals communicatedto effect a write operation, according to this preferred embodiment ofthe invention, and in a normal operating mode (i.e., a “legacy” mode).According to the architecture of FIG. 2, this data write operation is awrite of data to data register 14. As such, according to the preferredembodiment of the invention, a write to data register command (e.g.,command value 80H) is effected, in the manner shown in FIG. 4 a,following which the destination memory address within flash memorydevice 10 is communicated by controller 30, both prior to the writeoperation to now be descried relative to FIG. 4 c. To effect a datawrite operation, controller 30 drives command latch enable signal CLEand address latch enable signal ALE both inactive low, indicating toflash memory device 10 that input data to be written (i.e., neither acommand nor an address value) will be communicated on input/output linesI/O1 through I/On. Of course, chip enable signal CE_is also drivenactive low for this operation. Controller 30 then issues active lowpulses of the write enable signal WE_along with each byte or word ofdata presented on input/output lines I/O1 through I/On. In thisembodiment of the invention, as in the case of command and addresstransmissions, the valid input data is presented synchronous with therising edge of write enable signal WE_at the end of each pulse. Inresponse to this edge, flash memory device 10 latches the then-currentstate of input/output lines I/O1 through I/On, corresponding to a wordor byte of input data, either into data latch within I/O control circuit20, or directly (or eventually, as the case may be) over bus DATA_BUS todata register 14. FIG. 4 c illustrates the communication of four wordsD_(in)(0) through D_(in)(3) over input/output lines I/O1 through I/On,synchronous with four pulses of write enable signal. WE_.

FIG. 4 d illustrates the operation of controller 30 and flash memorydevice 10 in carrying out a data read operation (from flash memorydevice 10 to controller 30) in a normal operating (“legacy”) modeaccording to the preferred embodiment of the invention. As in the caseof a data write operation, a command sequence (e.g., as shown in FIG. 4a) and an address sequence (e.g., as shown in FIG. 4 b) has previouslybeen carried out prior to this read operation. One or more writeoperations may also have been previously carried out before this read(i.e., in which case, if the read is to the same address as has justbeen written, this read would serve as a verify of the previous write),or a write operation (e.g., as shown in FIG. 4 c) may be performed afterthis read operation, in the form of a read-modify-write sequence to thesame memory address. In response to the communication of an addressprior to a read, the contents of the memory cells corresponding to thataddress are sensed, and forwarded to data register 14. As such, the readoperation of FIG. 4 d is a read of the current comments of data register14. And in order to effect this read operation, controller 30 issues theappropriate command (e.g., command E0 h) in a command operation in themanner described above relative to FIG. 4 a.

In this operation, as in the data write operation, controller 30 hasdriven command latch enable signal CLE and address latch enable signalALE both inactive low, and has driven chip enable signal CE_ active low.Controller 30 indicates a desired read operation by taking write enablesignal WE_ inactive high. In this data read operation, flash memorydevice 10 outputs data words D_(out) in response to falling edges ofactive low pulses of read enable signal RE_, as generated by controller30. In this normal operating mode, therefore, controller 30 is able tosynchronize the receipt of data from flash memory device 10 by issuingan active low pulse of read enable signal RE_, and then waiting aspecified access time (permitting flash memory device 10 to effect someor all of the operations involved in sensing the states of its memorycells, and in forwarding the sensed states to its data register 14 andout to input/output lines I/O1 through I/On. Controller 30 can thenlatch into its input buffer the data states of input/output lines I/O1through I/On to receive the data from flash memory device 10. In theexample of FIG. 4 d, four data words D_(out)(0) through D_(out)(3) areread in sequence; a rising edge of chip enable signal CE_ ends this readoperation, following which flash memory device 10 causes its outputdrivers in I/O control circuit 20 to place input/output terminals I/O1through I/On into a high-impedance (“high-Z”) state.

Other operations according to this normal operating (“legacy”) mode arealso preferably available, such operations as known in the art. Forexample, controller 30 can read the contents of status register 24 inthis normal operating mode by issuing a specific status command (e.g.,command code 70H) following the timing of FIG. 4 a, and receiving thecontents of status register 24 over input/output lines I/O1 through I/Onin response to its issuing of an active low pulse of read enable signalRE_.

As evident from FIGS. 4 c and 4 d, one data word or byte (referred to as“data word” in the following description) is communicated for each cycleof write enable signal WE_ or read enable signal RE_, as the case maybe. And as evident from these Figures and the foregoing description,controller 30 controls and sources both the write enable signal WE_ andthe read enable signal RE_ in this normal operating mode. In the readoperation, particularly, because only one data word is read for eachcomplete cycle of read enable signal RE_, controller 30 has sufficienttime, according to conventional flash memory timing requirements andperformance, to itself issue its read data strobe (read enable signalWE_) and to receive and latch the read data. However, this level ofperformance may not necessarily be sufficient for high-speed use offlash memory device 10, such as when flash memory card 25 is used asmass data storage in a computer system. In addition, it is contemplatedthat this “legacy” mode of operation may not be able to keep up with thehigh speed external interface mode from controller 30 to the hostsystem, such as under the UDMA interface protocol mentioned above.

According to the preferred embodiment of the invention, therefore, flashmemory device 10 provides an advanced, higher performance, read andwrite operating mode, and controller 30 is constructed to take advantageof that advanced mode. The operation of flash memory device 10 andcontroller 30 in utilizing this advanced mode will now be described indetail, in connection with the flow diagrams of FIGS. 5 a and 5 b, andthe timing diagrams of FIGS. 6 a through 6 e.

FIGS. 5 a and 6 a through 6 c illustrate the operation of flash memorydevice 10 in performing a data read operation (i.e., from flash memorydevice 10 to controller 30, in flash memory card 25). In process 40 ofFIG. 5 a, flash memory device 10 and controller 30 are powered-up, whichplaces both of these devices into the normal operating mode (process42), as described above relative to FIGS. 4 a through 4 d. In process44, read and write operations (if any) in this normal mode are carriedout in this normal operating (“legacy”) mode.

Entry into the advanced read operating mode begins with process 46, inwhich controller 30 issues a memory address value to flash memory device10 according to the normal operating mode, as described above relativeto FIG. 4 b. The memory address issued by controller 30 in process 46 isthe initial memory address from which data will be read in this advancedoperating mode, and preferably follows the transmission of thecorresponding read address input command as described above. In process48, controller 30 issues an “initiate data transfer”, or “IDT”, commandsequence to flash memory device 10. FIG. 6 a illustrates this operationin further detail.

According to the preferred embodiment of the invention, an “IDT” commandis issued by controller 30 to flash memory device to initiate theadvanced data transfer mode in process 48. This command is issued in asimilar manner as the issuing of commands described above relative toFIG. 4 a, with controller 30 driving chip enable signal CE_ active low,address latch enable signal ALE inactive low, and command latch enablesignal CLE active high. The rising edge of an active low pulse of writeenable signal WE_ serves as the data strobe for the IDT command valueIDT_CMD (which is a binary word having a value different from otherassigned command values) driven onto input/output lines I/O1 throughI/On by controller 30. After a specified time following write enablesignal WE_ being taken high, controller 30 places input/output linesI/O1 through I/On into a high-impedance state. And, after anotherelapsed time t_(rel) following the rising edge of the write enablesignal WE_, when strobing in the IDT command, controller 30 then alsoreleases control of its read enable signal RE_, permitting control logic18 of flash memory device 10 to drive the state of corresponding lineRE_ (without risking data contention with controller 30).

Once the IDT command has been latched into and executed by flash memorydevice 10, flash memory device 10 then begins executing high-speed moderead data transfer process 50. As shown in FIG. 6 a, this read datatransfer process begins with flash memory device 10 issuing the firstvalid output data word D_(out)(0), following the elapse of a non-zeroaccess time after the rising edge of write enable signal WE_. Once itpresents this first output data word D_(out)(0), flash memory device 10then begins issuing active pulses of read enable signal RE_, synchronouswith additional output data words D_(out)(1) et seq. According to thispreferred embodiment of the invention, one data word D_(out)(k) isissued synchronously with each edge, falling and rising, of the readenable signal RE_driven by flash memory device 10 itself. In the exampleof FIG. 6 a, each output data word D_(out)(k) follows its strobe edge bya non-zero access time; alternatively, each read enable signal RE_ edgemay be issued (or delayed to be issued) to controller 30 within itscorresponding valid data word D_(out)(k).

According to the preferred embodiment of the invention, therefore, therate at which flash memory device 10 presents data to controller 30 viainput/output lines I/O1 through I/On, in this advanced mode, issubstantially faster than the data rate in the normal operating mode(FIG. 4 d), approaching twice the data rate in typical realizations.This higher data rate is in part enabled by permitting flash memorydevice 10 to issue the read data strobe edges of read enable signal RE_,which eliminates the propagation delay and necessary timing windows thatwould be involved if controller 30 were issuing these read data strobeedges.

As will be apparent to those skilled in the art, however, the increasedrate at which output data is presented on input/output lines I/O1through I/On, with all other factors equal, substantially increases thepower dissipation within flash memory card 25, primarily from the outputdrive circuit within I/O control circuit 20 of flash memory device 10 inthis read operation. This power consumption is exacerbated as the dataword width (i.e., the number n of input/output lines I/O1 through I/On)increase, as is the modern trend. According to the preferred embodimentof the invention, this power consumption is greatly reduced by reducingthe voltage swing of the output signals on input/output lines I/O 1through I/On as will now be described.

Conventional flash memory devices utilize the well-known 3.3 volt busstandard, in which the minimum high level output voltage V_(OH) is 2.4volts and the maximum low level output voltage V_(OL) is 0.4 volts, andfor which the nominal voltage swing is about 3.3 volts. According tothis standard, as well known in the art, these voltages are based on apower supply voltage that is nominally at 3.30 volts, and which has aspecification range of between 2.70 volts and 3.60 volts. According tothe conventional normal operating mode for modern flash memory devices,the output data rate is 25 MHz (i.e., a data transition every 40 nsec);in the worst case of a given input/output line I/Ok making a datatransition in each cycle, flash memory device 10 would be required tocharge the capacitance of input/output line I/Ok at a frequency of 12.5MHz. Assuming a typical line capacitance of 65 pF for this input/outputline I/Ok, the current consumption, in milliamperes, for oneinput/output line I/Ok can be calculated from:

I _(k) =f*C(V _(OH) −V _(OL))

which, for this example, results in:

I _(k)=12.5*0.065(3.3)=2.681 mA

using the typical 3.3 volt swing between the high and low data levels.The current consumed in driving read enable signal RE_ will be twicethat of current I_(k), as it must charge its corresponding conductor forevery transition. The overall current consumed in the conventionalnormal operating mode for this example, assuming eight input/outputlines I/O1 through I/O8, will therefore be:

I _(total)=8(2.681)+2(2.681)=26.81 mA

According to the preferred embodiment of the invention, the bus voltageis substantially reduced from this conventional 3.3 v bus level, forexample to a bus voltage of about 1.8 volts, defining the nominalvoltage swing at about 1.80 volts. In this case, an example of theminimum high output level voltage V_(OH-R) limit may be about 1.44 volts(80% of the nominal power supply voltage) and an example of the maximumlow output level voltage V_(OL-R) may be about 0.36 volts (20% of thenominal power supply voltage). In this reduced voltage operation, thesevoltages are based on a power supply voltage that is nominally at 1.80volts, and which is permitted to range from about 1.60 volts to about2.0 volts. Assuming the best case (for data transfer; worst case forcurrent consumption) data rate of 50 MHz, the charging frequency forinput/output lines I/O1 through I/On will be 25 MHz. The current I_(k)consumed for a single input/output line I/Ok can thus be calculated:

I _(k)=25*0.065(1.8)=2.925 mA

using the typical 1.8 volt swing between the high and low data levels.This current consumption, per input/output line, is thus not muchdifferent for the advanced operating mode, while providing twice thedata rate. However, read enable signal RE_is operating at the samefrequency as in the normal operating mode (but clocking one data wordwith each edge, rather than only with the rising edge). But its voltageswing is of course also reduced, as it is also operating at the 1.8 voltbus voltage; as such, its current consumed is the same as that for oneof the input/output lines. The overall current consumed in the advancedoperating mode for this example, again assuming eight input/output linesI/O1 through I/O8, will therefore be:

I _(total)=8(2.925)+1(2.925)=26.33 mA

which is slightly less than that for conventional flash memory cardsoperating at the 3.3 volt bus voltage. And because this slightly lowercurrent consumption is attained at a lower voltage swing (1.8 voltsversus 3.3 volts) for the input/output signals, the power consumed inthis advanced operating mode is substantially lower than that consumedin conventional flash memory cards. According to these examples, thepower consumed in a conventional eight I/O flash memory in the normaloperating mode will be about 88 mW (3.3 volts times 26.81 mA), while thepower consumed by the example of the preferred embodiment of theinvention described above will be about 47 mW (1.8 volts times 26.33mA). This substantial reduction in power consumption is attained incombination with a substantial improvement in data transfer rate,approaching doubling of the data rate for large bursts.

According to the preferred embodiment of the invention, therefore, inwhich the advanced read data transfer is executed at these lower busvoltages (relative to conventional flash memory devices), the currentconsumed in the advanced mode is no worse than that consumed byconventional flash memory devices in the normal operating mode. And,according to this preferred embodiment of the invention in which flashmemory device 10 has the capability of operating in the normal operatingmode as well, the lower bus voltage is used in both the advanced modeand the normal operating mode, as well as for other operations includingthe communication of commands and address value. As such, flash memorydevice 10 consumes less power, in its transfer of data, thanconventional flash memory devices.

As evident from the foregoing, command and address signals arecommunicated in the normal operating mode. For ease of implementation,the bus voltage for the communication of these signals is alsopreferably maintained at the lower bus voltage (e.g., 1.8 volts),providing additional reduction in the power consumption of flash memorycard 25.

Referring back to FIG. 5 a, flash memory device 10 according to thisembodiment of the invention is capable of responding to a suspendrequest from controller 30. It is contemplated, according to thisinvention, that a suspend of the read data transfer may be deemednecessary by controller 30 for any one of a number of reasons, forexample if its internal receive data buffer is full. As such, decision51 of FIG. 5 a determines whether such a suspend is required. If not,the high-speed read data transfer continues in process 56, in the mannerdescribed above relative to FIG. 6 a.

If controller 30 requires a suspend of the read data transfer (decision51 is YES), it issues a suspend request in process 52. In this exemplaryimplementation, this request is made by controller 30 asserting anactive high level on address latch enable signal ALE during the readtransfer operation. FIG. 6 b illustrates such a suspend operation,occurring during a read data transfer in the advanced mode (i.e., afterthe mode has been invoked and the data transfer has begun). In theexample of FIG. 6 b, controller 30 requests the data transfer suspend byasserting address latch enable signal ALE during the data transfer fromflash memory device 10 to controller 30. In response. flash memorydevice 10 suspends read enable signal RE_ (either when at a low level asshown, or at a high level), and thus delays the issuing of a next dataword after read enable signal RE_ is suspended. Given the rapidswitching rate of read enable signal RE_ and input/output lines I/O1through I/On in this advanced mode, it is contemplated that one or twoadditional data words, and the corresponding edges of read enable signalRE_, may be driven by flash memory device 10 after address latch enablesignal ALE is driven active high to request the suspend. In thisexample, controller 30 has asserted address latch enable signal ALEduring output data word D_(out)(4), and flash memory device 10 respondsby holding further transitions of read enable signal RE_ and ofinput/output lines I/O1 through I/On during output data word D_(out)(6).

This suspension of further data transfer continues until controller 30executes process 54 to inactivate address latch enable signal ALE, thusending the suspend. As shown in FIG. 6 b, the suspend state is endedupon controller 30 taking address latch enable signal ALE to an inactivelow state. According to this embodiment of the invention, thistransition of address latch enable signal ALE serves as the read datastrobe for the next output data word from flash memory device 10, namelydata word D_(out)(7) in this example. After this initial post-suspenddata word, flash memory device 10 again generates the read strobe signalby asserting transitions of read enable signal RE_ as shown. The nexttransition of read enable signal RE_, in this example, is the strobe forthe second output data word D_(out)(8) following the end of the suspendperiod. The advanced mode read data transfer continues, in process 56,as shown in FIG. 6 b.

Referring back to FIG. 5 a, the advanced mode read data transfercontinues until such time as controller 30 wishes to terminate thetransfer, which it indicates to flash memory device 10 in processes 58,59. Typically, this transfer will be terminated upon controller 30determining that the end of a page within flash memory device 10 isbeing reached, although the transfer may also be terminated bycontroller 30 for other reasons (e.g., upon receiving all of the desireddata for the operation).

According to this example, to terminate this data transfer, controller30 first issues a suspend in process 58, for example by asserting anactive high level of address latch enable signal ALE as described above.FIG. 6 c illustrates this example of the termination processes 58, 59,within which is shown the transition of address latch enable signal ALEduring the advanced read data transfer operation. This suspend operationof process 58 is transformed into termination of the advanced read datatransfer by controller 30 performing process 59 during the suspendoperation. Alternatively, process 59 may be performed after flash memorydevice 10 itself determines that its output data has reached the end ofa page, in which case flash memory device 10 itself maintains readenable signal RE_ at its last level, and maintains the current (i.e.,last) output data word on input/output lines I/O1 through I/On; in thiscase, address latch enable signal ALE will remain inactive low. In theexample shown in FIG. 6 c, this data transfer is terminated bycontroller 30 asserting an active high level on command latch enablesignal CLE while address latch enable signal ALE is active high. Inresponse to this transition of command latch enable signal CLE, flashmemory device 10 controls its output drivers to place input/output linesI/O1 through I/On into a high impedance state, and also releases itscontrol of the conductor corresponding to read enable signal RE_, inboth cases permitting controller 30 to take control of these lines whenappropriate, while avoiding data contention issues. As shown in theexample of FIG. 6 c, because the suspend and terminate operationsoccurred with read enable signal RE_ at a low level, controller 30 willdrive an inactive high level on the corresponding line once it takescontrol of the read enable signal RE_, causing a transition as shown; ifthe suspend and terminate operations occurred with read enable signalRE_ already at a high level, there would of course be no transition onthis line.

Flash memory device 10 then returns to the normal operating mode(“legacy” mode), passing control back to process 44 in the flow diagramof FIG. 5 a. A new advanced mode read data transfer will require anotherinstance of initiate process 48, according to this preferred embodimentof the invention.

Further in the alternative, an unconditional termination will occur ifcontroller 30 de-asserts chip enable signal CE_. However, it iscontemplated that this uncontrolled termination could result in“glitches” and other spurious and unspecified events both internally toand externally from flash memory device 10 and controller 30.

According to this preferred embodiment of the invention, an advanced,high performance, mode is also provided for the transfer of data fromcontroller 30 to flash memory device 10, in other words for write datatransfer operations. The flow diagram of FIG. 5 b, together with thetiming diagrams of FIGS. 6 a and 6 d through 6 e, illustrate thisoperation, as will now be described.

To effect the advanced mode write data transfer, flash memory device 10begins from the normal operating mode, as entered in process 60. As inthe case of the read data transfer, normal mode operations (if any) mayfirst be carried out, in process 62. In process 64, controller 30 issuesan address value to flash memory device 10, in this normal operatingmode as described above relative to FIG. 4 b. And in process 66,controller 66 initiates the advanced data transfer mode in similarfashion as performed for the advanced read data transfer described aboverelative to FIG. 6 a. It is contemplated that the write data transfer inthis advanced mode will be executed, through process 66, substantiallyidentically as was the read data transfer. As such, it is contemplated,for example, that the command value IDT_CMD issued in process 66 is thesame for both the read and write data transfer operations.Alternatively, separate command values may be assigned for the twooperations, relative to one another.

In process 68, controller 30 and flash memory device 10 carry out theadvanced write data transfer. FIG. 6 d illustrates the timing of signalsin an example of this operation, including process 66 in which thecombination of command value IDT_CMD, an active high level for commandlatch enable CLE, and an active low pulse for write enable signal WE_are issued by controller 30 to flash memory device 10, thus initiatingthe advanced mode data transfer. As in the previous example, addresslatch enable signal ALE is maintained at an inactive low level, and chipenable signal CE_ is maintained active low. And because this operationwill be a data write, read enable signal RE_ (not shown in FIG. 6 d)will be held inactive high by controller 30 throughout. Because writedata transfer process 68 remains under the full control of controller30, in this embodiment of the invention, the latency between the issuingof the command IDT_CMD and the beginning of the write data transfer canbe much shorter than that prior to the first output data word in theread data transfer (FIG. 6 a). Preferably, a specified time elapsesbetween the rising edge of the pulse of write enable signalWE_corresponding to the initiation command IDT_CMD, and the falling edgeof the first pulse of write enable signal WE_corresponding to the firstinput data word D_(in)(0) as shown.

Once the write data transfer begins, in this preferred embodiment of theinvention, both the falling and rising edges of write enable signal WE_serve as write data strobes, asserted by controller 30. As shown in FIG.6 d, this permits controller 30 to issue a new valid write data wordD_(in)(k) onto input/output lines I/O1 through I/On synchronously witheach edge of write enable signal WE_. As a result, the write datatransfer rate in this advanced mode can approach twice the data rate ofa normal operating mode write operation, for the same write enablesignal WE_ frequency.

According to this embodiment of the invention, referring back to FIG. 5b, suspend decision 69 is also performed throughout the advanced modewrite data transfer. Typically, the need for a write suspend isdetermined solely by controller 30, it being contemplated that flashmemory device 10 can receive input data at this data rate without bufferoverflow, etc. If no suspend is necessary (decision 69 is NO), the datatransfer continues in process 72. If controller 30 requires it (decision69 is YES), the suspend of the write data transfer is effected inprocess 70. In this example, suspend process 70 is effected simply bycontroller 30 extending the state of write enable signal WE_ as long asnecessary. This suspension may be performed in either state (writeenable signal WE_ held high or held low); FIG. 6 d illustrates suspendprocess 70 during the duration of write data word D_(in)(2), in whichwrite enable signal WE_ is held low. Of course, controller 30 does notissue additional write data words D_(in)(k) during suspend process 70.The end of the suspend period is effected merely by controller 30driving a transition of write enable signal WE_, together with the nextvalid write data word D_(in)(3) in the example shown in FIG. 6 d, tocontinue the write data transfer (process 72).

And, as in the read data transfer case, the voltage levels of the dataand control signals (input/output lines I/O1 through I/On, and the linefor write enable signal WE_) are preferably at a lower voltage levelthan the conventional levels, for example with a 1.8 volt “swing”between the high and low logic levels. As discussed above in detail,this lower voltage bus will maintain the power consumed by this advancedwrite data transfer mode at or below that consumed in conventional flashmemory systems operating in the normal operating mode, at half the datarate.

Referring back to FIG. 5 b in combination with FIG. 6 e, termination ofthe write data transfer is effected in the identical manner as thetermination of the read data transfer. In process 74, controller 30asserts address latch enable signal ALE to an active high level inprocess 74 to suspend the transfer, and then asserts command latchenable signal CLE to an active high level (while maintaining addresslatch enable signal ALE high) in process 76, which then terminates thewrite data transfer. FIG. 6 e illustrates the timing of the varioussignals in terminating a write data transfer. Write enable signal WE_iseither held at a high level as shown in FIG. 6 e, or taken to a highlevel from a low level after the last data word D_(in)(5) (in thisexample) has been latched. Following termination of the advanced modewrite data transfer, effected by holding the high levels at address andcommand latch enable signals ALE, CLE, respectively, for a specifiedpulse width, the normal operating mode of flash memory device 10 andcontroller 30 is then again entered.

In this example, the normal operating mode is effectively a “default”operating mode, considering that the execution of a command is requiredto invoke the advanced mode, and considering that the operation of flashmemory device 10 returns to the normal operating mode at the terminationof the data transfer (i.e., without requiring execution of a command).Alternatively, flash memory device 10 may be configured so thatexecution of a command is required to enter both the advanced datatransfer mode and the normal operating mode, such that once flash memorydevice 10 is in the advanced data transfer mode, it will remain in thatmode until a command to return to the normal operating mode is issued bycontroller 30 and executed by flash memory device 10. Of course, such anapproach involves additional overhead in the nature of commandsequences.

Further in the alternative, it is contemplated that the “default”operating mode of flash memory device 10 may be the advanced datatransfer mode, such that all data transfers are performed in theadvanced mode unless a command is issued by controller 30 to place flashmemory device 10 into the normal operating mode. According to thisalternative embodiment of the invention, once flash memory device 10 isin the normal operating mode, completion of the data transfer willresult in flash memory device 10 returning to the advanced data transfermode.

FIG. 7 illustrates the operation of flash memory device 10 according tothis alternative preferred embodiment of the invention, in which theadvanced data transfer mode is effectively a “default” mode. In process80, flash memory device 10 and controller 30 are powered-up, orotherwise complete a reset operation, and enter the advanced operatingmode in process 82, effectively as a default condition, withoutrequiring issuance or execution of a command. In process 84, read andwrite operations are carried out in the advanced data transfer operatingmode, effectively as described above relative to FIGS. 6 b through 6 d.In this advanced mode, it is contemplated that suspend operations andthe like may be carried out as described above relative to FIG. 6 c, forexample; it is further contemplated that other operations, such asaddress, command, and status communication operations, may still followthe normal operating mode approach if desired.

In process 86, flash memory card 25 according to this preferredembodiment of the invention prepares for a normal, or “legacy”, datatransfer by controller 30 issuing an address value to flash memorydevice 10, indicating the memory location at which the normal operatingmode transfer is to begin. In process 88, controller 30 issues a commandsequence to initiate the normal operating mode; it is contemplated thatthis command sequence will substantially correspond to that describedabove relative to FIG. 6 a, with the command sequence itself preferablyoperating according to the normal operating mode (contemplated that thecommand code value is a single byte value). In response to command 88,flash memory device 10 performs a normal operating mode read or writedata transfer operation, for example as described above relative toFIGS. 4 c and 4 d, depending upon the direction of the data transfer. Inthis embodiment of the invention, controller 30 preferably issues boththe read data strobe and the write data strobe clock signals, asdescribed above.

And, according to this alternative preferred embodiment of theinvention, the normal operating mode is exited upon completion of thedata transfer. In the example of FIG. 7, similarly as described aboverelative to FIGS. 5 a and 5 b, termination of the data transfer bycontroller 30 issuing a suspend signal (e.g., an active level on theaddress latch enable signal) in process 92, followed by controller 30terminating the transfer in process 93 (e.g., by issuing an active levelof the command latch enable signal). Upon termination of the normaloperating mode data transfer according to this preferred embodiment ofthe invention, control returns to process 84, in which the advanced datatransfer mode is again entered, and read and write data transferoperations carried out as desired in process 84.

It is contemplated that other alternative approaches to entry andexiting the various operating modes of flash memory device 10, beyondthis alternative preferred embodiment of the invention, will be apparentto those skilled in the art having reference to this specification, andalso that these and such other alternative implementations are withinthe scope of this invention as claimed.

Referring now to FIGS. 8 a through 8 e, the timing of signals betweenflash memory device 10 and controller 30 in the context of flash memorycard 25, according to a second preferred embodiment of the inventionwill now be described in detail. The overall processes of entry into,exit from, and operation during the advanced mode according to thissecond preferred embodiment of the invention preferably follow theprocesses described above relative to FIG. 5 a for a read operation, andto FIG. 5 b for a write operation. As such, a detailed description ofthose processes will not be repeated here in connection with FIGS. 8 athrough 8 e.

As described above relative to the first preferred embodiment of theinvention, flash memory device 10 and controller 30 are in the normaloperating (“legacy”) mode following power-up. As such, read and writeoperations (if any) in this normal mode are carried out as desired bythe user. Entry into the advanced operating mode for a read operation isthen performed by controller 30 issuing a memory address value,corresponding to the initial memory address from which data will be readin this advanced operating mode, to flash memory device 10 in the normaloperating mode. As before, this memory address is placed on input/outputlines I/O1 through I/On in combination with an active level on theaddress latch enable signal. ALE.

After the memory address is communicated, controller 30 issues an“initiate data transfer”, or “IDT”, command sequence to flash memorydevice 10 as before, by chip enable signal CE_ active low, address latchenable signal ALE inactive low, and command latch enable signal CLEactive high. FIG. 8 a illustrates this operation. The rising edge of anactive low pulse of write enable signal WE_ serves as the data strobefor the IDT command value IDT_CMD (which is a binary word having a valuedifferent from other assigned command values) driven onto input/outputlines I/O1 through I/On by controller 30. After a specified timefollowing write enable signal WE_ being taken high, controller 30 placesinput/output lines I/O1 through I/On into a high-impedance state.

According to this second preferred embodiment of the invention, flashmemory device 10 will take control of, and drive, the read enable strobesignal RE_. Accordingly, as shown in FIG. 8 a, upon time t_(rel)elapsing after the rising edge of the write enable signal WE_ thatstrobes the IDT command, controller 30 releases control of its readenable signal RE_. Control logic 18 of flash memory device 10 can thendrive the state of corresponding line RE_ without contending withcontroller 30. Flash memory device 10 then begins performing thehigh-speed, advanced mode, read data transfer. According to this secondpreferred embodiment of the invention, as shown in FIG. 8 a, flashmemory device 10 presents the data from the addressed memory cells at ahigher data rate than in the legacy mode, in combination with a higherfrequency read enable signal RE_ than is available in the legacy mode.

For example, flash memory device 10 may present the output data atinput/output lines I/O in this advanced mode at twice the frequency atwhich data is presented in the legacy mode, in combination with itsdriving of the read enable signal RE_ at twice the frequency of thatsignal in the legacy mode. For the example in which the maximumavailable data rate and read strobe frequency in the legacy mode is 25MHz, the advanced mode data rate and the frequency of read enable signalRE_ can be as high as 50 MHz. Because flash memory device 10 is itselfsourcing read enable signal RE_ and also the data words, the frequencieswith which these signals are produced by flash memory device 10 is notunder the direct control of controller 30.

FIG. 8 a illustrates this advanced mode read operation. The read datatransfer process begins with flash memory device 10 issuing the firstvalid output data word D_(out)(O), following the elapse of a non-zeroaccess time after the rising edge of write enable signal WE_. After thefirst output data word D_(out)(0), flash memory device 10 then beginsissuing active pulses of read enable signal RE_, synchronous withadditional output data words D_(out)(1) et seq. According to thispreferred embodiment of the invention, one data word D_(out)(k) isissued synchronously with each full cycle of the read enable signal RE_.In the example of FIG. 8 a, the falling edge of read enable signal RE_is the signal edge with which the data words are synchronous; of course,the rising edge of read enable signal RE_ (i.e., read enable signal“RE”) may instead be the operative edge. As shown in FIG. 8 a, eachoutput data word D_(out)(k) follows its corresponding falling edge ofread enable signal RE_ by a non-zero access time. Alternatively, eachfalling edge of read enable signal RE_ may be issued (or delayed to beissued) to controller 30 within its corresponding valid data wordD_(out)(k).

According to this second preferred embodiment of the invention,therefore, the rate at which flash memory device 10 presents data tocontroller 30 via input/output lines I/O1 through I/On, in this advancedmode, is substantially faster than the data rate in the normal operatingmode (FIG. 4 d), approaching twice the data rate in typicalrealizations. This higher data rate is in part enabled by permittingflash memory device 10 to issue the read data strobe edges of readenable signal RE_, which eliminates the propagation delay and necessarytiming windows that would be involved if controller 30 were issuingthese read data strobe edges. In addition, as described above inconnection with the first preferred embodiment of the invention, thisincreased data rate at input/output lines I/O1 through I/On is attainedwithout dramatically increasing the power consumption of flash memorydevice 10 and controller 30, by using a reduced voltage swing of thedata signal (and of read enable signal RE_, if desired). As describedabove, the nominal voltage swing of these lines is substantially reducedfrom this conventional 3.3 v bus level, for example to a nominal voltageswing at about 1.80 volts.

For the example of a sixteen-bit input/output bus interface betweenflash memory device 10 and controller 30 (i.e., there are sixteeninput/output lines I/O1 through I/O16), this second preferred embodimentof the invention involves only slightly more power consumption than thatof the first preferred embodiment of the invention. As described above,at data rate of 50 MHz, the charging frequency for input/output linesI/O1 through I/On will be 25 MHz according to this preferred embodimentof the invention. The current I_(k) consumed for a single input/outputline I/Ok can thus be calculated:

I _(k)=25*0.065(1.8)=2.925 mA

using the typical 1.8 volt swing between the high and low data levels.However, because read enable signal RE_ is operating at twice thefrequency as in the normal operating mode and in the first preferredembodiment of the invention, and as such its current consumption will betwice that of a single input/output line I/Ok:

I _(RE)=50*0.065(1.8)=2*2.925 mA=5.850 mA

Accordingly, the overall current consumed in the advanced operating modefor this example, for the case of sixteen input/output lines I/O1through I/O16, will therefore be:

I _(total)=16(2.925)+5.850=52.65 mA

which is slightly more than what would be consumed according to thefirst preferred embodiment of the invention (i.e., 49.73 mA). Thecurrent consumed for conventional data transfer, for the case of asixteen-bit I/O bus, would be, following the description above:

I _(total)=16(2.681)+2(2.681)=48.62 mA

which is slightly lower than the 52.65 mA according to this secondpreferred embodiment of the invention. However, even though the currentconsumed according to this embodiment of the invention is slightlyhigher than the conventional implementation, this current level isattained at a lower voltage swing (1.8 volts versus 3.3 volts) for theinput/output signals. As a result, the power consumed in this advancedoperating mode is substantially lower than that consumed in conventionalflash memory cards. According to these examples, the power consumed in aconventional sixteen I/O flash memory in the normal operating mode willbe about 160 mW (3.3 volts times 48.62 mA), while the power consumed bythe example of the preferred embodiment of the invention described abovewill be about 95 mW (1.8 volts times 52.65 mA). This substantialreduction in power consumption is attained in combination with asubstantial improvement in data transfer rate, approaching doubling ofthe data rate for large bursts.

As before, controller 30 may issue a suspend request to flash memorydevice 10, for example if its input buffer (from flash memory device 10)fills. The operation of flash memory device 10 in response to such asuspend request is shown in FIG. 8 b. The suspend request is made bycontroller 30 asserting an active high level on address latch enablesignal ALE during a high-speed read transfer. In response to thisrequest, flash memory device 10 suspends read enable signal RE_ (eitherwhen at a high level as shown, or at a low level), delaying the nextcycle of read enable signal RE_. Because of the high data rate transfer,one or two additional data words, and the corresponding cycles of readenable signal RE_, may already be in the output “pipeline” of flashmemory device 10, so that these corresponding data words may be outputbefore flash memory device 10 reacts to the suspend request. In thisexample, controller 30 has asserted address latch enable signal ALEduring output data word D_(out)(4), and flash memory device 10 respondsby holding further cycles of read enable signal RE_and transitions atinput/output lines I/O1 through I/On during output data word D_(out)(6).

FIG. 8 c illustrates the termination of the advanced mode, high-speedread data transfer according to this preferred embodiment of theinvention. As before, controller 30 terminates the high-speed datatransfer by first issuing a suspend request by asserting an active highlevel of address latch enable signal ALE. During the suspend request,controller 30 terminates the data transfer operation by asserting anactive high level on command latch enable signal CLE while address latchenable signal ALE is active high. In response, flash memory device 10causes its output drivers to place input/output lines I/O1 through I/Oninto a high impedance state, and also releases its control of theconductor corresponding to read enable signal RE_. Controller 30 may nowtake control of these lines if appropriate for the next operation.

Write operations in the advanced mode, according to this secondpreferred embodiment of the invention, will now be described relative toFIGS. 8 d and 8 e. As in the case of the read data transfer, theadvanced mode is entered after flash memory device 10 and controller 30have operated in the normal, or legacy, mode. Entry into the advanceddata transfer mode for a write operation is performed similarly as forthe advanced read data transfer described above relative to FIG. 8 a. Asshown in FIG. 8 d, the advanced mode command value IDT_CMD is issued bycontroller 30 in combination with an active high level for command latchenable CLE, and an active low pulse for write enable signal WE_. Asbefore, address latch enable signal ALE is maintained at an inactive lowlevel, and chip enable signal CE_ is maintained active low. The datawrite operation for this entry into the advanced mode is indicated bycontroller 30 maintaining read enable signal RE_ (not shown in FIG. 8 d)inactive high throughout the write operation. A specified time is thenpermitted to elapse between the rising edge of the pulse of write enablesignal WE_with the initiation command IDT_CMD, and the falling edge ofthe first pulse of write enable signal WE_ corresponding to the firstinput data word D_(in)(0).

During this advanced mode write data transfer, according to this secondpreferred embodiment of the invention, the frequency of cycles of writeenable signal WE_ is increased, for example to twice the frequency ofthat used in writes in the normal mode. Falling edges of write enablesignal WE_ serve as write data strobes in this example. And also, inthis advanced mode as well as in the normal mode, write enable signalWE_ is asserted by controller 30, in combination with the data valuesdriven by controller 30 on input/output lines I/O1 through I/On. Asshown in FIG. 8 d, controller 30 issues a new valid write data wordD_(in)(k) onto input/output lines I/O1 through I/On synchronously witheach falling edge of write enable signal WE_. Because the frequency ofwrite enable signal WE_ is doubled, in this example, the write datatransfer rate in this advanced mode can approach twice the data rate ofa normal operating mode write operation. For example, if the maximumwrite data transfer rate, and write enable signal frequency, is 25 MHzin the normal mode, the data transfer rate and write enable signalfrequency may be increased to as high as 50 MHz in the advanced modeaccording to this second preferred embodiment of the invention. Flashmemory device 10 is constructed, according to this embodiment of theinvention, so that it can receive and process data at that higher rate.Of course, controller 30 may use an actual write enable signal frequencyand data rate at a frequency lower than the maximum (e.g., 50 MHz),depending on the system application and the rate at which controller 30is itself processing data.

Also according to this embodiment of the invention, a suspend may beinserted into the advanced mode write data transfer. In this example, asbefore, controller 30 suspends the write data transfer simply byextending the state of write enable signal WE_ as long as necessary, asshown in FIG. 8 d. This suspension may be performed in either state(write enable signal WE_ held high or held low). New data wordsD_(in)(k) are not issued, of course, during this suspend period. Thesuspend period is ended by controller 30 driving the next cycle of writeenable signal WE_, together with the next valid write data wordD_(in)(3) in the example shown in FIG. 8 d, to continue the write datatransfer.

And, as in the read data transfer case, the voltage levels of the dataand control signals (input/output lines I/O1 through I/On, and the linefor write enable signal WE_) are preferably at a lower voltage levelthan the conventional levels, for example with a 1.8 volt “swing”between the high and low logic levels. As discussed above in detail,this lower voltage bus will maintain the power consumed by this advancedwrite data transfer mode at or below that consumed in conventional flashmemory systems operating in the normal operating mode, at half the datarate.

Referring now to FIG. 8 e, the advanced mode write data transfer isperformed identically as the termination of an advanced read datatransfer according to this second preferred embodiment of the invention.Termination of the advanced mode is performed by controller 30 drivingaddress latch enable signal ALE to an active high level to suspend thewrite transfer, during which time controller 30 asserts command latchenable signal CLE to an active high level (while maintaining addresslatch enable signal ALE high) to terminate the write data transfer.Write enable signal WE_ is either held at or taken to a high level afterthe last data word D_(in)(5) (in this example) has been latched.Following termination of the advanced mode write data transfer, effectedby holding the high levels at address and command latch enable signalsALE, CLE, respectively, for a specified pulse width, the normaloperating mode of flash memory device 10 and controller 30 is then againentered.

According to this second preferred embodiment of the invention,therefore, the advanced, or high-speed, data transfer mode of operationis carried out in an alternative manner, by permitting the use of higherfrequency strobe signals to increase the data rate. It is contemplatedthat this operation, according to the second preferred embodiment of theinvention, may be more compatible with the desired operation in someflash memory applications.

Flash memory device 10, controller 30, and flash memory card 25according to the preferred embodiments of the invention thus provideimportant advantages over conventional devices and systems. Thisinvention enables high data transfer rates, approaching double the datarate of conventional devices and systems, while still providing commandand signal compatibility with “legacy” devices that do not have theadvanced capability. In addition, the lower bus voltage signals involvedin the advanced data transfer mode maintain the overall device andsystem current and power consumption at near, or even below, that ofconventional flash memory devices and systems.

As a result, it is contemplated that this invention can especially bebeneficial in those digital system applications in which data transferrates are especially critical. One such application, as discussed above,is in high-performance digital still cameras. In such cameras, the imageresolution (and thus the data captured per image) now exceeds 10megapixels, with up to 12.4 megapixel cameras now available on themarket. The data transfer rate from the image sensor to flash memory iscritical, however, as this data transfer rate is a direct factor in rateat which images can be captured, commonly experienced as “shutter lag”by the camera user. And because the camera user is concerned primarilywith the absolute delay encountered (i.e., independent of the amount ofdata acquired in each image), as the image resolution increases, thisload on the data transfer rate is exacerbated. Another potentialapplication for such high data transfer rates is in the use ofsolid-state flash memory as mass storage media in computer systems,substantially replacing some or all of the magnetic disk drive massstorage conventionally used. The ability to use solid-state memory,rather than disk drives, is contemplated to enable furtherminiaturization and portability of computer systems, and also to greatlyincrease the functionality of modern portable and handheld systems.

While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

1. A method of operating a flash memory device to communicate with aflash memory controller, comprising the steps of: in a normal operatingmode, presenting data words to the controller over input/output linesresponsive to transitions of a selected polarity of a read data strobesignal received from the controller, the read data strobe signal havinga maximum available frequency in the normal operating mode; executing acommand received from the controller to initiate an advanced datatransfer mode; then driving the read data strobe signal to thecontroller; and synchronously with transitions of a selected polarity ofthe read data strobe signal and in the advanced data transfer mode,presenting data words over input/output lines to the controllercorresponding to data stored in the flash memory device; wherein theread data strobe signal in the advanced data transfer mode has afrequency higher than the maximum available frequency in the normaloperating mode.
 2. The method of claim 1, wherein the normal operatingmode corresponds to standardized specifications for communicationsbetween flash memory devices and controllers, the standardizedspecifications including a first voltage specification defining high andlow logic levels for the read data strobe signal and the data words overthe input/output lines; and wherein the presenting, driving, andpresenting steps are performed using a second specified voltagespecification defining high and low logic levels for the read datastrobe signal and the data words over the input/output lines, the highand low logic levels in the second specified voltage specificationdefining a substantially smaller voltage swing than that defined by thehigh and low logic levels in the first specified voltage specification.3. The method of claim 2, wherein the voltage swing defined by the highand low logic levels in the first specified voltage specification isnominally about 3.3 volts; and wherein the voltage swing defined by thehigh and low logic levels in the secondly specified voltagespecification is nominally about 1.8 volts.
 4. The method of claim 1,further comprising: after the executing step, receiving write datastrobe signals from the controller; and responsive to receivingtransitions of a selected polarity of the write data strobe signal,latching in data words on the input/output lines for storing in theflash memory device wherein the write data strobe signal has a maximumavailable frequency in the normal operating mode; and wherein the writedata strobe signal in the advanced data transfer mode has a frequencyhigher than the maximum available frequency in the normal operatingmode.
 5. The method of claim 4, further comprising: in the normaloperating mode, latching in data words on the input/output lines forstoring in the flash memory device responsive to transitions of a firstpolarity of the write data strobe signal received from the controller.6. The method of claim 1, wherein the executing step is performedresponsive to receiving an initiate command value on the input/outputlines in combination with a transition of a first polarity of a writedata strobe signal from the controller, and in combination withreceiving a command latch enable signal from the controller.
 7. Themethod of claim 6, further comprising: during the step of presentingdata words to the controller in the advanced data transfer mode, andresponsive to receiving a suspend request from the controller, holding avalue of a data word on the input/output lines and holding a currentstate of the read enable signal.
 8. The method of claim 7, furthercomprising: responsive to receiving an end of the suspend request fromthe controller, resuming the step of presenting data words to thecontroller in the advanced data transfer mode and the step of drivingthe read data strobe signal.
 9. The method of claim 7, wherein thesuspend request corresponds to receiving a transition of a controlsignal from the controller.
 10. The method of claim 9, furthercomprising: before the steps of presenting data words to the controllerin the advanced data transfer mode and driving the read data strobesignal, receiving a memory address from the controller over theinput/output lines in combination with a transition of a first polarityof a write data strobe signal from the controller, and in combinationwith receiving an address latch enable signal from the controller;wherein the suspend request corresponds to a transition of the addresslatch enable signal during the step of presenting data words to thecontroller in the advanced data transfer mode.
 11. The method of claim10, wherein the end of the suspend request corresponds to a secondtransition of the address latch enable signal.
 12. A method of operatinga flash memory device to communicate with a flash memory controller,comprising the steps of: in an advanced data transfer operating mode:driving the read data strobe signal to the controller and synchronouslywith transitions of a selected polarity of the read data strobe signal;and presenting data words over input/output lines to the controllercorresponding to data stored in the flash memory device; executing acommand received from the controller to initiate a normal operatingmode; and then presenting data words to the controller over input/outputlines responsive to transitions of a selected polarity of a read datastrobe signal received from the controller; wherein the read data strobesignal has a maximum available frequency in the normal operating mode;and wherein the read data strobe signal in the advanced data transfermode has a frequency higher than the maximum available frequency in thenormal operating mode.
 13. The method of claim 12, wherein the normaloperating mode corresponds to standardized specifications forcommunications between flash memory devices and controllers, thestandardized specifications including a first voltage specificationdefining high and low logic levels for the read data strobe signal andthe data words over the input/output lines; and wherein the driving, andpresenting steps are performed using a second specified voltagespecification defining high and low logic levels for the read datastrobe signal and the data words over the input/output lines, the highand low logic levels in the second specified voltage specificationdefining a substantially smaller voltage swing than that defined by thehigh and low logic levels in the first specified voltage specification.14. The method of claim 13, wherein the voltage swing defined by thehigh and low logic levels in the first specified voltage specificationis nominally about 3.3 volts; and wherein the voltage swing defined bythe high and low logic levels in the secondly specified voltagespecification is nominally about 1.8 volts.
 15. The method of claim 12,further comprising: in the advanced data transfer mode, receiving writedata strobe signals from the controller; and responsive to receivingtransitions of a selected polarity of the write data strobe signal,latching in data words on the input/output lines for storing in theflash memory device. wherein the write data strobe signal has a maximumavailable frequency in the normal operating mode; and wherein the writedata strobe signal in the advanced data transfer mode has a frequencyhigher than the maximum available frequency in the normal operatingmode.
 16. The method of claim 15, further comprising: in the normaloperating mode, latching in data words on the input/output lines forstoring in the flash memory device responsive to transitions of a firstpolarity of the write data strobe signal received from the controller.